reduce default maxInactiveAge to 15s (#29817)
reduce default maxInactiveAge to 15s (#29817)

With persistent caching and other disposing fixes we can safely reduce the time before pages are disposed.

The last 2 client pages are still never disposed.
zeit/next.js,azukaru/next.js,zeit/next.js,azukaru/next.js,azukaru/next.js,JeromeFitz/next.js,JeromeFitz/next.js,azukaru/next.js,JeromeFitz/next.js,JeromeFitz/next.js,zeit/next.js